In general, ball screws are mechanical elements that convert a rotation of a screw shaft or nut into an axial translation of a mechanical part. The ball screw includes a screw shaft formed with a helical screw groove on its outer circumferential surface. A nut is formed with a helical screw groove on its inner circumferential surface. A number of balls are rollably contained within a ball rolling passage formed by the oppositely arranged screw grooves. In general, ball screws used for industrial machines and positioning mechanisms have a structure that mainly supports only a pure thrust load. Accordingly, radial loads and moment loads are directly applied to a nut and a screw shaft in such a ball screw. For example, when these ball screws are solely used without using any linear guide, other structural parts are used separately from the ball screw due to a caused imbalance in the inside supporting load distribution.
However, it is common in ball screws used in automobile actuators that a compound load, including thrust loads, radial loads and moment loads, is applied to the ball screw to correspond to requirements of structural limitations in the actuator to reduce its size and manufacturing cost. One example of this includes an actuator used for a variable valve mechanism and a transmission of an engine.
It is believed that the durability of the ball screw is appreciably reduced due to an increase in the contacting surface pressure caused by unevenness of inside load distribution of ball screw. Thus, the number of loading balls is reduced when a compound load, which includes radial and moment loads, other than a pure thrust load is applied to the ball screw. A representative ball screw is shown in FIG. 5.
The ball screw 51 has a screw shaft 52 with a helical screw groove 54 formed on its outer circumferential surface. A nut 53 is formed with a helical screw groove 55 on its inner circumferential surface corresponding to the screw groove 54. A number of balls 56 are rollably contained within a ball rolling passage formed by the oppositely arranged screw grooves 54, 55. Bridge members 57 connect the screw grooves 55 of the nut 53. Since the bridge members 57 are arranged at opposite ends of the nut 53, so that they are inclined at about 90° toward the circumferential direction relative to a direction along which the moment load is generated, it is possible to eliminate balls being arranged at the axially central portion of the nut 53. Thus, it is possible to reduce the manufacturing cost of the ball screw and to improve its durability due to an increase of the load supporting capacity of both the radial and moment loads of the ball screw 51 (see Japanese Laid-open Patent Publication No. 169740/2004).
Although the ball screw 51 of the prior art is effective when the directions of the generated radial load and moment load are constant, it is believed that when plural directions of the generated radial load and moment load exist or when the ball screw 51 is used in a manner so that the nut 53 is rotated not only the effect cannot be expected but the durability is reduced.